Essentially two basic processes are known for producing hollow plastic parts.
A first process is disclosed in U.S. Pat. No. 4,101,617. In this process, the melted plastic or so-called plastic melt is injected into the cavity of the injection mold in a first step. Thereafter, gas under pressure is injected through the same nozzle through which the plastic melt was introduced. The gas under pressure is introduced into the mass which is still formable such that the plastic mass fills out the mold and becomes rigid under the gas pressure which is introduced directly into the melt and maintained. Thereafter, the gas pressure is reduced to atmospheric pressure for opening the mold and the completed hollow body can be removed.
Another process is disclosed in U.S. Pat. No. 4,740,150 which is different from the above method in that the plastic melt and the gas (nitrogen or air) are introduced at different locations of the mold by means of separate injection devices.
In the second process, the injection of the pressure gas takes place via an injection device independently of the feed system for the plastic melt. In this process, the gas-injection device is mounted independently of the plastic feed system at another location of the injection mold especially suitable for metering the gas. This is especially advantageous for the production of hollow bodies having large or complicated cross sections. However, a disadvantage is associated with this process of separate injection of the plastic melt and gas especially with respect to plastic melts made of polyamide. The disadvantage is that the plastic melt cools relatively rapidly after being introduced into the mold and in this way forms a skin at the region of the walls of the hollow injection mold through which a rapid and loss-free introduction of premetered gas quantities into the hollow mold is hardly possible. If, however, the premetered gas quantity introduced into the mold for maintaining a pregiven inner gas pressure is not complete (that is not without loss), then fluctuations occur in the wall thickness of the hollow body to be manufactured as well as uneven surfaces which causes the manufactured body to be unusable and therefore leads to scrap. Depending upon the configuration of the injection mold for the plastic hollow body to be produced, the gas-injection nozzle for introducing the gas cannot penetrate the melt skin, which has already formed, without difficulty. The skin can easily bulge outwardly when inserting the needle-like gas-injection nozzle whereby the skin lifts inwardly with an edge layer off the wall of the mold so that the injected gas only partially flows into the inner hollow mold and with delay; whereas, another portion of the gas quantity flows outwardly and is lost with respect to the injection process. This occurrence is especially disadvantageous when utilizing plastics which are partially crystalline such as the above-mentioned polyamide.